This invention relates to fluid-gauging systems and fluid-gauging methods.
The invention is especially, but not exclusively, concerned with fuel-gauging systems for aircraft.
Measurement of the quantity of fuel within an aircraft's fuel-tanks is commonly performed by means of one or more capacitive probes arranged for immersion in any fuel present. The capacitance of the probe varies in accordance with the depth of fuel in the tank, thereby enabling an indication of fuel level to be obtained. Changes in permittivity of fuel which would affect the capacitance of the probe can be compensated for by use of a permittivity cell (which may be in the form of a parallel-plate capacitor of open construction) mounted at the bottom of the fuel-tank, so as always to be immersed in any fuel present. An indication of volume may be obtained directly if the tank is of a regular shape, that is, if the volume of fuel present varies in a linear fashion with the depth of fuel. For irregularly shaped tanks the probe may be characterised, that is, the probe plates may be suitably-shaped such that the surface area of the plates covered by fuel varies in a non-linear fashion with depth but in a manner that is directly related to the volume of fuel present. Alternatively, the output of the probe may be supplied to a computer in which is stored a model of the fuel-tank from which can be obtained an indication of the volume with knowledge of the fuel depth.
In many applications, such as, for example, in aircraft, it is necessary to have an indication of the fuel mass rather than its volume. Although the mass of fuel can be readily determined by measuring its density with some form of densitometer, the cost of densitometers capable of providing the high accuracy needed in some applications can be very high. Most modern aircraft are equipped with several fuel-tanks and, because of the variations in density between different fuels, such as might be supplied to different tanks during refuelling stops at different airports, it is necessary to obtain a measure of the density of each of the different fuels and fuel mixtures within the different tanks. The cost of providing a fuel-gauging system having a densitometer mounted in each of an aircraft's fuel-tanks will therefore be correspondingly high, and, in many cases prohibitive.
To avoid the need to use densitometers, it has been proposed instead to measure the permittivity of fuel with a relatively simple and inexpensive capacitive permittivity cell and to derive the density from the permittivity using a relationship of the form:
D.sub.K =(K-1)/0.763(0.7+0.2(K-1)) . . . (I)
where
D.sub.K is the density of the fuel; and
K is the permittivity of the fuel.
Since the permittivity cells can be relatively inexpensive, it is possible to mount a cell in every tank and thereby compensate for fuels of different densities. The above relationship, however, is only approximate; random variations from fuel to fuel mean that errors of up to about 2% can be experienced. In the future, with fuels derived from an increasing number of sources, it is likely that even greater errors may be experienced. Although a fuel-gauging system of such limited accuracy may be satisfactory in some cases, the need to carry more fuel than is absolutely necessary, in order to allow sufficient safety margin for inaccuracies in the system, will mean that the payload that can be carried by the aircraft must be correspondingly reduced.